The
optical performance
of an ultra high speed ICCD camera
|
An
intensified CCD ultra high speed camera is mostly used to take images
under
residual
light conditions or with extremely short exposure times. Although the
ICCD camera provides an image
intensifier, in some cases the resulting
image quality will depend on the absolute light level available.
If
there is no light, there will be no image at all. If there is only a
very small quantity of light, it appears that light consists of
individual particles. There will not be a continuos illumination but a
"hail like" bombardment by single photons. Thus, at very low
illumination levels there will not be enough photons for the human eye
to form an image. Increasing the illumination will increase the number
of photons and a noisy image will appear. However, it will still not
be possible to identify very small details until the illumination
increases even more as shown in the diagram. This means the image
quality, i.e. the resolution
of the image, generally depends on the
available light level in low light conditions. These working conditions
are called
the low light level regime or shot
noise limited
regime and the image
quality is described by the signal to noise
ratio SNR. |
|
If there is enough light, the noisiness will
disappear. The image
quality
will be much better and will no longer depend on the
illumination intensity. As can be seen from the diagram, the
optical resolution in linepairs/mm remains constant in high light level
conditions. Therefore, the image quality may be described by this limiting
optical resolution, by means of the modulation
transfer
function MTF or the contrast
transfer function CTF.
|
|
Please
note:
Granulous image quality under very low light conditions is not a
deficiency of the ICCD camera system, but the manifestation of the
quantum character of light. This may only be overcome by increasing the
available light level, e.g. by using longer exposure times or, even
better, by using sophisticated frame
adding techniques as the STANFORD COMPUTER OPTICS Dynamic
Range
Expansion System. |
|
|
|
